The fan case of a turbo-fan gas turbine engine performs several functions in association with the rotating fan in operation. The aerodynamic function of the fan case is to direct the axial flow of air in conjunction with the fan into the bypass duct and compressor of the engine core.
Typically, the fan directs a primary air stream through the compressor and turbines of the engine and secondary airflow through an annular radially outward bypass duct. For the aerodynamic function of the fan case, it is essential that the clearance between the rotating fan blades and the internal surface of the fan case be kept within an acceptable range to maximize the fan efficiency.
Accordingly, it is common practice to line the internal air path surfaces of the fan case with an abradable material. On initial operation of the engine and rotation of the newly manufactured fan, some of the abradable material is rubbed off on contact with the tips of the rotating fan blade to establish a minimal tip gap. During the high speed rotation of the fan, the fan blades stretch elastically due to the centrifugal force subjected upon them. The dynamic stretching causes the tips of the metallic blades to come in contact with the abradable liner and consequently abrade the abradable material. Due to manufacturing tolerances, each fan blade will have its unique variation, and the actual degree of running clearance required and stretching of blades will vary to some degree between different fans. The provision of abradable material therefore allows for minimizing the clearance between the fan blade tips and the annular internal air path surface of the fan case.
During storage, or when an aircraft on which one or more gas turbine engines are installed is not in operation, it is always preferable to shield the inlet of the engine so as to prevent water or any foreign objects from entering. An example of a shield used for this purpose is shown in U.S. Pat. No. 6,871,819 to Garric. Garric discloses a protection device for an air intake structure of a jet engine installed on an aircraft. In particular, this device may be assembled on the air intake structure during long term aircraft storage phases in order to avoid the entry of any foreign elements inside the engine, through the ventilation scoop, the exhaust orifices or the intake of the fan case ducting.
However, such maintenance procedures are not always strictly followed, and therefore, it is sometimes possible for water to penetrate the gas turbine engine and form a pool of water in the fan case. During cold weather conditions, this water pool can freeze. If any fan blade tips are in contact with the water pool when it freezes, the ice can restrain the fan from freely rotating. Since the abradable liner of the fan case has a relatively rough and porous surface, the frozen pool of water strongly adheres to the liner. Consequently, the amount of force required to remove or break the ice can be significant depending on the size of the pool. Therefore, the ice restraint on the fan blade tips can be strong enough to prevent the fan from turning during start up. As a result this can prevent the turbo-fan engine from starting and/or possibly cause engine damage.
Typically, in order to prevent water from pooling in the fan case, drainage features have been fashioned in the form of simple drainage holes drilled through the fan case. However, these drilled drainage holes compromise the structural integrity of the fan case. Particularly, when the drainage holes are located in a critical containment region of the case, the case must be reinforced by increasing the thickness of the fan case in that area. This results in a significant weight and cost increase of the fan case.
Therefore, it is desirable to provide a fan case that is adapted to prevent such water pool and ice from forming on the abradable liner of the fan case wherein the structural integrity of the fan case is not compromised and no significant weight and/or cost increase penalty is incurred.
Features that distinguish the present invention from the background art will be apparent from review of the disclosure, drawings and description of the invention presented below.